Subscriber multiplexer, a telephone system and a method for communicating the condition of a subcriber interface

ABSTRACT

A subscriber multiplexer includes a receiver for receiving signals transmitted from subscriber interfaces a feeder and for feeding those signals further to a telephone exchange via a respective PCM connection, and a feeder for feeding signals transmitted from the telephone exchange via the respective PCM connection further to the subscriber interfaces. For detecting the condition of the subscriber interfaces, the subscriber multiplexer includes measuring devices for checking the condition of the subscriber interface of the subscriber multiplexer, and a controller for activating the measuring devices in response to a measurement request signal transmitted by the telephone exchange via the respective PCM connection to achieve a measurement result, and for transmitting a message describing the measurement result to the telephone exchange to a telephone system and a method for communicating the condition of a subscriber interface of a subscriber multiplexer.

This application is the national phase of international applicationPCT/F195/00346 filed Jun. 14, 1995 which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to a subscriber multiplexer comprising means forreceiving signals transmitted from subscriber interfaces and feedingthose signals further to a telephone exchange via a PCM connection,means for feeding signals transmitted from the telephone exchange via aPCM connection further to subscriber interfaces, and measuring devicesfor checking the condition of a subscriber interface of the subscribermultiplexer. The invention further relates to a telephone systemcomprising a telephone exchange, and a subscriber multiplexer connectedto the telephone exchange by a PCM connection, the multiplexercomprising subscriber interfaces and measuring devices for checking thecondition of subscriber interfaces by means of check measurements. Theinvention further relates to a method for communicating to the telephoneexchange the condition of the subscriber interface of a subscribermultiplexer connected to the telephone exchange via a PCM connection.

A subscriber multiplexer is previously known, the subscriber interfacesof which are connected to subscriber stations, such as telephones andsimilar devices, by means of a converter means and two-wire lines. Theconverter means adapts, in that case, the subscriber stationelectrically to the subscriber multiplexer, and carries out thenecessary analog-to-digital conversions. The subscriber stationmultiplexer is connected to the telephone exchange via a PCM (Pulse CodeModulation) connection, in which one channel of the PCM connection isallocated for each subscriber interface. The frame structure of the PCMconnection between the subscriber multiplexer and the telephone exchangeis typically in accordance with CCITT (The International Telegraph andTelephone Consultative Committee) recommendation G.704, the interface ofthe subscriber multiplexer and the telephone exchange typically being aninterface in accordance with CCITT recommendation Q.512.

The weakness of the above mentioned subscriber multiplexer is thedifficulty in monitoring the condition of subscriber interfaces, sincemonitoring the condition of a wire connected to a subscriber interfacerequires the visit of a maintenance person in site. The maintenanceperson connects a separate service terminal including measuring devicesto the subscriber multiplexer, subsequent to which the maintenanceperson detects the condition of the subscriber interface, e.g. by meansof measurements of current, voltage and/or insulating resistance.Checking the condition of the interface of a subscriber multiplexer isthus a highly inconvenient and slow measure, and also relatively costly,as it requires a visit of a maintenance person to the site of theinterface.

SUMMARY OF THE INVENTION

The object of this invention is to solve the above mentioned problemsand to provide a subscriber multiplexer that allows remarkably simplechecking of the condition of its subscriber interfaces. This is achievedwith a subscriber multiplexer of the invention, which is characterizedby comprising a control means for activating measuring devices inresponse to a measurement request signal transmitted by a telephoneexchange via a PCM connection, and for transmitting a message describinga measurement result to the telephone exchange, that message informingthe telephone exchange that the measured subscriber interface is inorder, or that it is faulty.

The invention further relates to a system in which the subscribermultiplexer of the invention may be utilized. The system of theinvention is characterized in that the subscriber multiplexer comprises:a control means which is adapted to transmit a first message to thetelephone exchange and to activate measuring devices in response to themeasurement request signal transmitted by the telephone exchange, toachieve a measurement result, and adapted to transmit a second messagedescribing the measurement result to the telephone exchange if thesubscriber interface is in order, and to wait for a time-limit signaltransmitted from the telephone exchange prior to transmitting the secondmessage if the subscriber interface is out of order.

The invention further relates to a method by means of which the resultof the measurement may be transmitted from the multiplexer to thetelephone exchange. The method of the invention is characterized in thatthe measurement request signal received from the telephone exchange isacknowledged by transmitting the first message to the telephoneexchange, and it is communicated that the subscriber interface is inorder by transmitting the second message₋₋ to the telephone exchange, orit is communicated that the subscriber interface is out of order bytransmitting the second message to the telephone exchange onlysubsequent to receiving the time-limit signal from the telephoneexchange.

The invention is based on the idea that checking the condition ofsubscriber interfaces is facilitated and accelerated significantly whenmeasuring devices are integrated to the subscriber multiplexer, by meansof which devices the operator may carry out check measurements ofsubscriber interfaces automatically from the operation and maintenancecentre of the telephone network, and receive a message indicating theresult of the measurement without this requiring a visit of amaintenance person to the subscriber multiplexer. The most remarkableadvantages of the subscriber station multiplexer, the system and themethod of the invention thus are that the condition of the subscriberinterfaces may be detected at remarkably lower costs, more rapidly andin a simpler way compared with prior art solutions, without thesignalling required for this remarkably complicating or loading thesignalling of the telephone exchange or the subscriber multiplexer.

In a preferred embodiment of the invention, the frame structure of thePCM connection connecting the telephone exchange and the subscribermultiplexer is in accordance with CCITT recommendation G.732, and theinterface between the telephone exchange and the subscriber multiplexeris a V2-type interface in accordance with CCITT recommendation Q.512,both as in effect as of Jun. 15, 1994. In this case, the signallingbetween the telephone exchange and the subscriber multiplexer pertainingto the measurement may be carried out in accordance with the inventioncompletely by using a single bit, i.e. bit c of time-slot T16corresponding to the subscriber interface to be measured in themultiframe structure, for which bit no specific use is defined in CCITTrecommendation G.732. Thus, the invention may also be utilized in priorart telephone systems in accordance with the letter recommendation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in closer detail bymeans of a preferred embodiment of the invention, with reference to theattached figures, in which

FIG. 1 illustrates a frame structure in accordance with CCITTrecommendation G.732 as in effect on Jun. 15, 1994,

FIG. 2 shows a block diagram of the system of the invention, and

FIG. 3 shows a flow diagram of the progress of the test in accordancewith the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a frame structure in accordance with CCITTrecommendation G. 732 as in effect on Jun. 15, 1994. Information to betransmitted both from the telephone exchange to the subscribermultiplexer and from the subscriber multiplexer to the telephoneexchange is transmitted by means of a frame structure in accordance withFIG. 1. It appears from FIG. 1 that one multiframe consists of sixteenframes, and that one frame comprises 32 time-slots. Each time-slot, inturn, comprises 8 bits. Information pertaining to channels 1 . . . 30 isset in time-slots 1 . . . 15 and 17 . . . 31. Time-slot 0 is used fortransmitting the synchronizing signal of the frame and time slot 16 isused for signalling.

It further appears from FIG. 1 that in time-slot 16 of frame 2,signalling pertaining to channels 1 and 16 is transmitted by means offour bits a-d. In accordance with the recommendations, the values ofthese four bits are set so that b=1, c=0 and d=1 when no signallingtakes place on these channels. It must be noticed that severalcountries, including Finland, have given recommendations of their ownregarding the values of the above mentioned bits when no signallingtakes place on a channel. For instance, according to the recommendationsgiven in Finland, the values of bits b, c and d are thus b=0, c=1 andd=1. The value of bit c is thus totally opposite to what is supposed inthe CCITT recommendation G.732 as in effect on Jun. 15, 1994. However,the invention will be described in the following in a case in accordancewith that version of that CCITT recommendation. The invention may,however, be applied in the Finnish telephone network with a smallmodification, i.e. by reversing the value of bit c from one to zero, orvice versa, in the following examples.

In accordance with the invention, bit c is utilized for detecting thecondition of the subscriber interfaces. In other words, if one wants todetect the condition of the subscriber interface corresponding to PCMchannel 1, bit c, which is circled in FIG. 1, is used for signallingbetween the telephone exchange and the subscriber multiplexer.

FIG. 2 shows a block diagram of the system of the invention. FIG. 2shows a telephone exchange 1 (e.g. NOKIA DX210 or DX220), to which twosubscriber multiplexers MUX1 and MUX2 are connected via terminals LT(Line Terminal). The subscriber multiplexers and the telephone exchange1 are connected via a 2 Mbit/s PCM connection, the frame structure ofwhich is in accordance with CCITT recommendation G.732, the framestructure between the telephone exchange and the subscriber multiplexersbeing a digital V2 interface in accordance with CCITT recommendationQ.512, both as in effect on Jun. 15, 1994.

In FIG. 2, thirty subscriber stations (telephones and similar subscriberstations) are connected with subscriber multiplexer MUX1 by means oftwo-wire lines and converter means (not shown in the figure) viainterfaces L1-L30. The converter means carry out necessaryanalog-to-digital conversions and adapt the two-wire lines electricallyto the subscriber multiplexer. There is a fixed 64 kbit/s channelavailable for each subscriber station connected to the subscriberinterface. Accordingly, telephones and additionally an analog privatebranch exchange 5, allocating for use two of the subscriber interfacesL1-L30 of subscriber multiplexer MUX2, are connected to subscribermultiplexer MUX2.

When the operator wants to detect, e.g. the operational condition of thesubscriber interface of subscriber multiplexer MUX1, the operatorinitiates manually a test cycle. In this case, the telephone exchange 1transmits a measurement request signal to subscriber multiplexer MUX1 bysetting bit c of time-slot T16 corresponding to subscriber interface L1to be measured in the multiframe structure, to the value 1. In otherwords, in this case bit c of time-slot T16 of frame 1 shown in FIG. 1 isused. Bit c is circled in FIG. 1 (the value of bit c is normally 0according to the above-identified CCITT recommendations). A control unit3 of subscriber multiplexer MUX1 detects that the value of that bit haschanged, as a result of which it activates a measuring device 2 andtransmits a first message V1 to the telephone exchange also by settingin the frames to be sent towards the telephone exchange bit c oftime-slot T16 corresponding to subscriber interface L1 to be measured inthe multiframe structure to the value 1.

Subsequent to this, the measuring device 2 measures the condition ofsubscriber interface L1 in a way known per se, e.g. by carrying outmeasurements of current, voltage and/or insulation resistance for thetwo-wire line, and by testing the condition of the converter means bymeans of a separate test program. The control unit 3 monitors theprogress of the measurements. When the measurements are completed, thecontrol unit 3 transmits the second message to the telephone exchange bysetting bit c of time-slot T16 corresponding to subscriber interface L1of the multiframe structure to the value 0 if subscriber interface L1 isin order according to the measurements. If, however, the measurementsindicate that the respective subscriber interface is faulty, the controlunit will wait until the telephone exchange transmits a time-limitsignal S2, before it transmits the second message V2 to the telephoneexchange 1.

Subsequent to transmitting a measurement request signal S1, thetelephone exchange 1 monitors the value of the bit c in the framesreceived from the subscriber multiplexer. When the telephone exchange 1detects that the value of that bit has changed to one (the first messageV1), it starts a timer. When the timer indicates that a certainpredetermined time has expired, the telephone exchange detects that thetest has failed, i.e. that the subscriber interface in question isfaulty (unless it has received a second message V2 by that time), and ittransmits a time-limit signal S2 to the subscriber multiplexer, bysetting the bit c to the value 0.

FIG. 3 shows a flow diagram of the progress of the test in a system ofthe invention. When the operator has started the test in block A, thetelephone exchange transmits a measurement request signal S1 in block B(i.e. bit c is set to the value 1) to the subscriber multiplexer. Thesubscriber multiplexer acknowledges the start of the test in block C bytransmitting a first message V1 to the telephone exchange (i.e. bit c isset to the value 1). This means that the measuring device of thesubscriber multiplexer starts testing the subscriber interface. When theexchange receives the first message, it initiates time supervision inblock D. The telephone exchange has received in advance the informationon how long the test is expected to last. If the time in questionexpires, the exchange states that the test has failed due to a faultysubscriber interface and transmits the time-limit signal S2 (i.e. bit cis set to the value 0) to the subscriber multiplexer in block E. Thesubscriber multiplexer acknowledges the time-limit signal with a secondmessage V2 (i.e. bit c is set to the value 0) in block F, subsequent towhich the test is ended in block G.

If, however, the measured subscriber interface of the subscribermultiplexer is in order, the subscriber multiplexer transmits the secondmessage V2 (i.e. bit c is set to the value 0) to the telephone exchangein block H. The telephone exchange acknowledges the second message inblock I by transmitting the timelimit signal S2 (i.e. bit c is set tothe value 0) to the subscriber multiplexer, subsequent to which the testis ended in block G.

Thus, the information on the condition of the subscriber interface ofthe subscriber multiplexer may be transmitted to the exchange byallocating a single bit per channel, i.e. bit c of time-slot T16. Theinformation received at the exchange only indicates whether thesubscriber interface is in order or not, after which more accurateinformation is obtained only when a maintenance person has tested thesubscriber multiplexer in site.

It should be understood that the above description and the figuresassociated therewith are only intended to illustrate the presentinvention. Different variations and modifications will be obvious forpersons skilled in the art, without deviating from the scope and thespirit of the invention set forth in the attached claims.

We claim:
 1. A telephone system, comprising:a telephone exchange havingan exchange portion of a PCM connection; a subscriber multiplexer forserving between said telephone exchange and a plurality of subscribers,comprising:a subscriber multiplexer portion of the PCM connection,constituting with said exchange portion respective PCM connection;receiving means for receiving signals transmitted by respective ones ofsubscriber interfaces, and for feeding those signals via said PCMconnection to said telephone exchange; feeding means for feeding signalstransmitted by said telephone exchange and received via said PCMconnection, to respective ones of said subscriber interfaces; an atleast one measurement device, said at least one measurement device beingarranged for checking a respective condition of any requester one ofsaid subscriber interfaces, and providing a respective measurementresult; a control means arranged for activating said at least onemeasuring device, in response to receiving a measurement request signaltransmitted by said telephone exchange via said PCM connection, forchecking the condition of a respective one of said subscriber interfacesto obtain the respective measurement result, and for transmitting arespective message to said telephone exchange via said PCM connection,based on said respective measurement result, that the respective one ofsaid subscriber interfaces is in order, or provides a respectiveconnection which is faulty; said telephone exchange being arranged toprovide, and said control means being arranged to wait to receive atime-limit signal before transmitting said respective message, if saidrespective measurement result indicates that the respective one of saidsubscriber interfaces provides a respective connection which is faultywherein said telephone exchange is arranged to provide said time-limitsignal when a predetermined time has passed since said telephoneexchange has sent said measurement request signal without thereafterhaving received said respective message, based on said respectivemeasurement result, that the respective one of said subscriberinterfaces is in order.
 2. The system of claim 1, wherein:saidsubscriber multiplexer and said telephone exchange are arranged tocommunicate on said PCM connection using a multiframe structure which isin accordance with CCITT recommendation G.732, over an interface whichis in accordance with a CCITT recommendation Q.512 for a V2 interface;and said control means is arranged to receive said measurement requestsignal and said time limit signal in a particular time slot of saidmultiframe structure, and to transmit said respective message based onsaid respective measurement result, in said particular time slot.
 3. Amethod for communicating to a telephone exchange whether any respectiveconnection of a plurality of connections of respective subscriberstations provided through respective subscriber interfaces of asubscriber multiplexer which is in turn connected by a PCM connection tothe telephone interchange is in order or is faulty, comprising the stepsof:the telephone exchange sending a measurement request signal to thesubscriber multiplexer via said PCM connection, thereby causing acontroller of the subscriber multiplexer to initiate operation of an atleast one measuring device for obtaining a measurement result indicativeof a condition of a respective connection provided via a respective oneof said subscriber interfaces and providing that measurement result tosaid telephone interchange; only if said measurement is indicative ofthe respective connection being in order, said controller sending afirst type of message to said telephone exchange via said PCMconnection; said telephone exchange waiting for a predetermined timefollowing sending said measurement request signal to receive arespective first type of message, and, if no such first type of messageis received upon expiration of said predetermined time, said telephoneexchange sending a time-limit signal to said controller via said PCMconnection, for thereby causing said controller to send to saidtelephone exchange via said PCM connection a second type of messageindicative of the respective connection being faulty.
 4. The method ofclaim 3, wherein:said telephone exchange and said subscribermultiplexer, when respectively sending said signals and said messages,communicating on said PCM connection using a multiframe structure whichis in accordance with CCITT recommendation G.732, over an interfacewhich is in accordance with a CCITT recommendation Q.512 for a V2interface, and to receive the respective of said measurement requestsignal and said time-limit signal in a particular time slot of saidmultiframe structure, and to transmit a respective message based on saidmeasurement result, in said particular time slot.
 5. The method of claim4, wherein:sending each of said signals and messages involves changingonly one data bit in a respective time slot of a respective frame.